1. Field of the Invention
The present invention relates to a hydraulic control apparatus for a hydraulic torque converter applicable to an automatic transmission for a vehicle. More particularly, the present invention relates to a hydraulic control apparatus for a hydraulic torque converter which improves cooling effect by recirculating fluid when slip start for operating a lock-up clutch (lock-up on) provided in a torque converter.
2. Description of Related Art
A torque converter applied to a vehicle includes a lock-up clutch for directly and mechanically transmitting torque.
The lock-up clutch is disposed between a front cover being an input-side rotation member of the torque converter and a turbine being an output-side rotation member of the torque converter.
A space between the front cover and the turbine is divided into a disengagement-side oil chamber close to the front cover and an engagement-side oil chamber close to the turbine.
Therefore, the lock-up clutch is operated or not by difference between hydraulic pressure of the disengagement-side oil chamber and hydraulic pressure of the engagement-side oil chamber.
That is, if fluid in the disengagement-side oil chamber is drained and fluid is supplied to the engagement-side oil chamber, the hydraulic pressure of the engagement-side oil chamber increases compared with the disengagement-side oil chamber. Therefore, a friction member of the lock-up clutch is coupled to the front cover by friction.
When operating the lock-up clutch, if friction coupling force of the lock-up clutch increases, the lock-up clutch is completely coupled to the front cover by friction. This state is called lock-up on state of the lock-up clutch.
At this time, if hydraulic pressure difference between the disengagement-side oil chamber and the engagement-side oil chamber is reduced by supplying fluid to the disengagement-side oil chamber, the lock-up clutch begins to slip.
If the lock-up clutch slips frequently, frictional heat may occur due to slip and durability of the friction member may be deteriorated due to thermal-degradation.
The information disclosed in this Background of the Invention section is only for enhancement of understanding of the general background of the invention and should not be taken as an acknowledgement or any form of suggestion that this information forms the prior art already known to a person skilled in the art.